You can also use a focusing hood as a direct-view finder.
Push down the front cover of it and hang it to a stopper inside, which give you a visual field of 80mm lens. Also pushing down the frame around the front cover gives you a visual field of 65mm lens.
If you use 105mm, 135mm, 180mm, 250mm lenses, you need to have the viewfinder masks for them (attached to the stopper which yellow arrows indicate in the right picture).